This invention relates generally to a voltage type pulse width modulation (PWM) converter/inverter system for converting waveforms of input and output currents to alternating or frequency waveforms, such as sinusodial waveforms, and to a method for controlling such a system. More particularly, this invention relates to control means for maintaining the direct current capacitor voltage constant not only during steady state conditions but also during transient state conditions.
Generally, a converter/inverter system is provided for converting an alternating current, such as in the case of commercial power, to a direct current by a converter. Then the direct current is smoothed by a smoothing circuit comprising a capacitor or the like for conversion back to an alternating current at a variable frequency determined by an inverter. PWM is one of the methods employed for controlling this type of system. In the sinusodial waveform, PWM control method, wherein waveforms of both input and output currents are converted to sinusodial waveforms, a converter and a direct current capacitor for performing the smoothing function generate a direct current of a constant voltage, i.e., a direct current power, from an alternating current of sinusodial waveform which is supplied to an inverter. The inverter converts this constant direct voltage into an alternating current frequency while controlling the pulse width uniformity whereby a power equal to the alternating current power of the sinusodial waveform is obtained.
The so-called feedback method of control has been employed for controlling the direct current capacitor voltage of a voltage PWM converter/inverter system, i.e., the terminal voltage of the smoothing capacitor. In this method, the direct current capacitor voltage is detected and its detected value is compared with a command or reference value to control the inverter. This control process is described in more detail, for example, in an article entitled, "High Performance AC Motor Speed Control System Using GTO Converters", T. Okuyama et al., Proceedings of IPEC, Tokyo '83, Institute of Electrical Engineers, April, 1983, pp. 720-731.
However, this conventional control process has a disadvantage in that it is difficult to control the direct current capacitor voltage, i.e., the voltage of the smoothing capacitor, to be maintained constant at the so-called transient state or stage, e.g., when the speed command of the motor load is abruptly changed, or there is an abrupt change in the load torque. Specifically, there is the danger that when power is regenerated, or when a load torque abruptly decreases, the direct current capacitor voltage rises, with an adverse result that switching devices employed in the converter and the inverter are damaged.
In the conventional control process, the feedback control is based on detection of only a change occurring relative to the terminal voltage of the direct current capacitor, and, accordingly, a detection delay is a disadvantage. In order to maintain the direct current capacitor voltage constant when there is an abrupt change in the load, it is necessary to set a large proportional gain, K.sub.p, and integral gain, K.sub.i, of the PI (proportional integral) controller employed in the feedback control system. However, large gains, K.sub.p and K.sub.i, often make the control system unstable, and, due to its stability, the gains, K.sub.p and K.sub.i, cannot be made too large.
Therefore, it is an object of this invention is to provide a voltage type PWM converter/inverter system to provide stable control at high speed for a direct current capacitor voltage between the terminals of the smoothing capacitor.
It is another object of this invention to provide a method for controlling a voltage PWM converter/inverter system resulting in continuous, controlled stability of the direct current capacitor voltage at high speed.